I got to the bottom of this, or rather the dealer did. It's a faulty MDI panel. This has now been replaced under warranty and it's all fine.
There's been a faulty batch of MDIs which are being replaced by Volvo as required - this is not however, an official recall. So no one will be contacted, and it's only when problems arise that you will be told, and I guess that's only if you go via a Volvo dealer.
This is a Volvo Penta D1-30F, manufactured in 2015, fitted in a boat commissioned in 2016. But this panel is fitted on a large range of engines.